Remove a lot of vec! macro invocations from Assemble

[Jasper-Bekkers]

This adds a assemble_into which just produces SPIR-V into a Vec<u32> instead of making lots and lots of new ones, this should reduce the assemble functions heap traffic quite a bit. This should address #96 and help me out at the same time.

[khyperia]

Would be nice to take a std::io::Write instead of a Vec, but this is better than before, so ✔️

[Jasper-Bekkers]

I thought of that - but io::Write (or io::Cursor) mostly operates on bytes so we'd just be doing a bunch of back and forth between u32 and u8's. Since SPIR-V is naturally expressed as u32's, this seemed like the easier / more ergonomic option.